Two new Elysia species were found in brackish mangrove habitats in the southern part of the Gulf of Thailand in the area around Pattani. Elysia bangtawaensis, a large species with grooved rhinophores, was discovered living on bare mud in mangrove forests. This species looks like a fallen leaf and shows sophisticated phototrophic behaviour enabling it to survive without feeding while producing many eggs over a long period. Elysia siamensis, a small species, was found on filamentous green algae in a brackish channel. The latter species has long, thin, cylindrical rhinophores, and is further characterized by a screw-like penis, and a long thin process on the tip of its metapodium.